Major Description
DistributedUpdateProcessor is currently very conservative in what params it will copy when creating requests for other nodes, using a hardcoded internal list of only 2 param names. There are good reasons for this behavior in general, but it's really limiting – especially when you're implementing a custom RunAlways UpdateRequestProcessorFactory (or an update processor that you expect to be configured after DUP).
This situation has come up for me in SOLR-445, so I'd like to add support for a new method that Factories can use to communicate with DUP regarding the params they expect it to forward.